The present invention relates to an overvoltage protecting element and, more particularly, to an overvoltage protecting element of gas-filled discharge tube type.
Overvoltage protecting elements of gas-filled discharge tube type are widely used to protect power transmission lines and electric equipment connected thereto from high voltage surges. In an overvoltage protecting element of this type, upon application of a high voltage surge, the initiating time of discharge may be delayed, the firing potential at the initiating time of discharge may vary, or the dielectric breakdown of the overvoltage protecting element may be caused due to the decrease in the dielectric strength. When these phenomena occur, the equipment to be protected may not be protected from the high voltage surge.
Various proposals have been made in order to solve these problems. For example, in order to prevent time lag of the initiation of discharge upon application of a high voltage surge, U.S. Pat. No. 3,588,576 proposes to form a conductive layer of lumped potential gradient in the vicinity of the main electrodes for discharging the overvoltage protecting element. According to this prior art, when the high voltage surge is applied to the overvoltage protecting element, the lumped potential gradient is obtained at the vicinity of the main electrodes by the conductive layer, so that discharge between the main electrodes may be facilitated. The overvoltage protecting element of gas-filled discharge tube type disclosed in this U.S.P. specification comprises a hollow cylindrical body of an insulator such as ceramics or glass, a pair of main electrodes which are hermetically sealed to both opening ends of the cylindrical body to define a sealed chamber therebetween and discharging surfaces of which oppose each other with a gap therebetween within the cylindrical body, and an elongated conductive layer which extends in the ionization area of the gap in the axial direction of the cylindrical body along the inner wall surface thereof. According to this U.S.P. specification, when a high voltage surge is applied to the overvoltage protecting element, the electric field lines may be lumped between the end parts of the elongated conductive layer and the opposing electrodes to ionize the gas in the ionization area of the gap at a high speed, so that the discharge between the main electrodes may be started earlier. However, in the overvoltage protecting element described in this U.S.P. specification, when dicharge of large currents is caused frequently between a pair of main electrodes incorporated in the overvoltage protecting element, part of the material constituting the main electrodes sputters and becomes attached to the inner wall surface of the cylindrical body surrounding these main electrodes. Then, a thin conductive layer is formed on the inner wall surface of the cylindrical body to be connected to the elongated conductive layer, and the dielectric strength of the element may be degraded. Due to this sputtering, the firing potential between the main electrodes may decrease or become unstable.
As an improvement over this overvoltage protecting element, there is known an overvoltage protecting element disclosed in U.S. Pat. No. 4,056,753 of the applicant of the present invention. An overvoltage protecting element disclosed in this U.S.P. specification comprises a hollow cylindrical body of an insulator, a pair of main electrodes which are hermetically sealed to both opening ends of the cylindrical body to define a sealed chamber therebetween and discharging surfaces of which oppose each other with a gap therebetween, and two conductive layers which are formed in the circumferential direction of the cylindrical body along the inner wall surface of the cylindrical body to oppose the main electrodes with a gap therebetween and which respectively have, at least at part thereof, projections protruding toward the opposing main electrodes. In the overvoltage protecting element described in this U.S.P. specification, the conductive layers and the main electrodes are capacitively coupled, so that the connection of the conductive layers and resultant degradation in the dielectric strength may be eliminated. However, since the two conductive layers respectively have projections extending toward the opposing electrodes, the manufacturing process may become complex in procedure. In addition to this, the discharge may be concentrated at these projections at the initial period of discharge, resulting in a decrease in the firing potential between the main electrodes and variations in performance.